High average power fiber lasers with diffraction-limited beams that are suitable for High Energy Laser (HEL) applications are currently primarily limited in output power by Stimulated Brillouin Scattering (SBS) and Modal Instability. Some known systems have been effective to scale power to greater than the 2 kW level by mitigating SBS. However, Model Instability may still bottleneck power-scaling in regular large-mode area (LMA) fiber, e.g., non-photonic-crystal fiber (PCF) and/or photonic bandgap (PBG) fiber. Specifically, Modal Instability may limit diffraction limited output power to a threshold near 2 kW, such as ˜2.2 kW for 20 μm core step-index dual-clad fiber laser.